«… mixing fork() and threads…»

I guess bliako is right. I had always a bad feeling with this idea. But I have another idea: You are on Fedora, right? Why don’t you simply fork? You can do this «by hand» or use MCE or Parallel::ForkManager. You know this. And as we now know your real command it would be also nice to see your real code. Else debugging is really hard and just a guessing game. One more thing: I don’t know anything about the lib(s) you use. Do you really need the full «framework»? And is it possible for you to port something you need from C to Perl? To get the idea you might take a look at List::MoreUtils::PP and compare it with List::MoreUtils. Best regards, Karl